1. Field of the Invention
The present invention is related to devices for biometric identification, and more specifically to an integrated multi-sensor device, such as a hybrid optical and electronic sensor, which may be employed for biometric identification, authentication, etc.
2. Description of the Prior Art
It is well known that no two person's fingerprints are identical. The differences between two individuals' fingerprints can therefore be used to identify and distinguish individuals for a variety of applications. Two primary categories of fingerprint sensing technology exist today: electrical and optical.
Broadly, optical sensing uses light (visible or otherwise) to capture an image of a user's fingerprint. For example, if a sufficiently intense light is incident on a user's finger, some of that light will be reflected. This reflection may be captured as an image and compared to a reference image to establish the identity of the user. There are many variations on this basic optical technology, including the selection of one or more different wavelengths of light to image the fingerprint, direct imaging, scanned imaging, total internal reflection (TIR) imaging, image enhancement (e.g., polarization), etc. In order to address difficulties such as dry, dirty or thickly callused skin, or fingertips worn smooth or damaged by surface cuts, scrapes, etc. which interfere with simply observing the surface pattern of an individual's fingerprints, certain optical fingerprint sensors examine attributes of a user's finger below the surface of the skin. See, e.g., Rowe et al., U.S. Pat. No. 6,560,352, which is incorporated by reference herein. Fundamentally, however, all optical technologies operate by capturing an image of a portion of a user's finger using light emitted from a light source.
Again broadly, electrical sensing captures an image of a user's fingerprint by measuring the response of one or more electrical components in the presence of the fingerprint. These responses are examined to determine characteristics of the surface and/or structure of a user's skin at or proximate the tip of the user's finger which are indicative of a fingerprint pattern. For example, capacitance is in part a function of the distance between two electrodes. If one electrode is a plate in the sensor and the other is a portion of a user's finger, as the distance between the finger and the plate vary the capacitance will vary. According to another example, the presence of a finger can alter the fringing effect between the two capacitor plates in a substrate. In either case, the height differences between a ridge and a valley (relative to the plate) in a fingerprint can be determined by observing the change in capacitance. An image of the fingerprint (or equivalently, regions thereof) can then be constructed. See e.g., Kramer, U.S. Pat. No. 6,512,381; Tartagni, U.S. Pat. No. 6,320,394; Knapp, U.S. Pat. No. 5,325,442; Tamori, U.S. Pat. No. 5,400,662; and Tamori, U.S. Pat. No. 5,429,006; each of which being incorporated by reference herein.
As each of the various optical and electrical sensing technologies have, to a greater or lesser extent, disadvantages which lead to errors in image construction and ultimately the possibility of misidentification a user, there is a virtually unending need to improve the accuracy of biometric sensing and identification. Furthermore, while no two individuals will have identical fingerprints, it is possible to construct a replica of an individual's fingerprint using a latent print or other captured form of an original fingerprint. Such replicated prints have historically been used to defeat the security of biometric identification systems, (so-called “spoofing”). Techniques for defeating spoofing, such as measuring temperature, electrical conductivity, optical reflectivity, etc. of the finger and proximate tissue are known (and referred to as “anti-spoofing”). However, virtually as soon as a new anti-spoofing technique is deployed, an effort surfaces to defeat it. Therefore, in addition to improved accuracy, there is a continuous need to improve anti-spoofing techniques.